1. Field of the Invention
This invention relates to the fast and accurate detection of faults on transmission lines in the presence of noise, and particularly to transients caused by Capacitive Voltage Transformers.
2. Description of Background
Protective relays are devices that are designed to identify and isolate failures in a power system. A failure often takes the form of an insulation breakdown (fault) that result in a change in the system voltage and/or current. Protective relays are applied in the power system in such a way that each relay is configured to detect failures within a specific portion of the power system commonly referred to as a zone.
Impedance relays respond to current and voltage as a function of the electrical impedance between the relay location and the location of the fault. The configuration parameter that defines the zone of a distance relay is commonly referred to as the reach. A protective relay should never respond to any event other than a fault within its particular zone. Further, the longer a fault persists in a power system, the greater the likelihood that the stability of the entire power system will be compromised. Therefore, a protective relay should be able to identify faults within its zone in a minimum possible time period.
Typically, in a microprocessor-based impedance relay, a discrete Fourier transform (DFT) calculates phasor values from samples of waveforms taken over a fixed period of time (a window). A DFT rejects harmonics of the fundamental frequency when taken over a full power cycle. The approach is problematic since the time that is required to detect a fault is a function of the length of the Fourier window, thus a shorter window generally produces a faster operating time. However, as the length of a window is shortened it becomes more difficult to discriminate between the fundamental frequency component and other components. For example, in the instance that a window length is shortened to a half (½) power cycle the DFT rejects only odd harmonics.
Accurate determination of the fault location typically requires the extraction of the fundamental frequency phasor components of a post-fault voltage and current. However, the post-fault voltage and current will contain other components. Further, a decaying DC component will exist in the current signals due to the point-on-wave at which the fault occurs and the inductive time constant of the system. Similarly, Capacitive Voltage Transformers (CVTs), arc resistance variations, shunt capacitance, and traveling waves effects also generate transients that negatively impact the phasor estimation process.
CVTs create a particular challenge for fast impedance protection functions, particularily under so called high Source to Impedance Ratios (SIRs). During faults when the input CVT voltage undergoes abrupt changes in its magnitude, the output CVT voltage used by protective relays includes significant transients associated with energy stored in the internal components of the CVT that need to re-adjust for a different input voltage level. These transients can reach 20-50% of the nominal voltage in magnitude and be relatively close to the nominal system frequency. This makes them very difficult to filter out particularly within the short time period in which protective relays are expected to operate.
Under high SIRs the steady state voltage measured by a protective relay for faults at the boundary of the protection zone can be very low, as low as 3-5% of the nominal value. With the CVT transients reaching 20-50% and the signal of interest dropping to 3-5%, the noise-to-signal ratio can be as high as 10. Not only is the noise very high, but its frequency spectrum is very close to the signal of interest at least for 1-2 power cycles in which the relay is expected to operate.
One method of dealing with the CVT transient is to insert a filter into the voltage signal path that is an inverted representation of the CVT transfer function. This removes the distortion generated by the CVT resulting in a signal that is an accurate reproduction of the power system voltage. This method performs optimally only when the filter coefficients reflect the parameters of the particular CVT that is connected to the relay.
Another approach is to apply a short Fourier window with a correspondingly reduced reach at fault inception and to increase both the window length and zone of coverage throughout the duration of the fault up to some fixed limit. While this approach can produce faster operation times for faults located close to the relay, it does not improve the performance throughout the zone protected by the relay. A detection algorithm can also be based on a model of the power system. In particular, a series R-L model of the faulted transmission line implies that the voltage and current must satisfy a first order differential equation.
Presently, there exists a need for a solution that relates to an impedance algorithm that can identify faults within its zone by processing samples of the waveforms in the time domain—without the need of a DFT. Further, the time required for detection should be less than one power cycle for faults throughout much of the zone of protection.